Manufactured seed technology allows the mass production of substantially identical plants without the time and labor-intensive exercise of sexual reproduction. Manufactured seed technology has been described in various patents assigned to the Weyerhaeuser Company of Federal Way, Washington, including U.S. Pat. No. 5,236,469, “Oxygenated Analogs of Botanic Seed”; U.S. Pat. No. 5,427,593, “Analogs of Botanic Seed”; U.S. Pat. No. 5,451,241, “Oxygenated Analogs of Botanic Seeds”; U.S. Pat. No. 5,701,699, “Manufactured Seed with Enhanced Pre-Emergence Survivability”; and U.S. Pat. No. 6,119,395, “End Seals for Manufacturing Seed.” Plants produced by this technology are genetically tailored to grow optimally in a particular locale and are capable of possessing certain other desirable traits through an in vitro culture of somatic or zygotic plant embryos.
Genetically tailored seeds, when compared to those produced by sexual reproduction, are desirable because sexual reproduction is often subject to genetic recombinational events that may result in variable traits in the progeny. Plant embryos created by in vitro cultures, however, lack the natural protective and nutritive features of natural botanic seeds; thus manufactured seed technology provides both protection and nutrition to plant embryos cultured in a laboratory. Without the protective features afforded by manufactured seeds, in vitro cultures lack shelter from the harsh soil environment and nutrients for survival during the critical stages of sowing and germination.
The coating of a manufactured seed is an important protective feature. Some embodiments of manufactured seeds utilize a seed coat made from wax impregnated paper having a wax component comprised of nine parts paraffin such as that available from Koster Keunen, Inc., Watertown, Conn. (CAS #8002-74-2), and one part carnauba wax, also available from Koster Keunen, Inc. (CAS #8015-86-9) (i.e., a 9:1 ratio of paraffin to carnauba). This wax mixture, when used in connection with a manufactured seed coat, has been observed by the present inventors to crack at temperatures around 1° C. and below. Cracks or microfissures in the manufactured seed coat leave the embryo susceptible to attack by microbes, fungi, and bacteria. Moreover, microfissures or cracks leave the plant embryo susceptible to desiccation and mechanical damage all of which reduce the seed's chances for successful germination.
Microcracking of the manufactured seed coat at lower temperatures poses an even greater challenge to successful germination in light of recently improved methods for long-term storage disclosed in U.S. Pat. No. 5,666,762, “Respiration-Limited Manufactured Seed,” and U.S. Pat. No. 5,732,505, “Manufactured Seed Comprising Desiccated and/or Frozen Plant Tissue,” also assigned to the Weyerhaeuser Company. These improved methods of long-term storage may require exposing manufactured seeds to temperatures at or below freezing for an extended period of time.
Several wax formulations exist that will resist cracking at temperatures below about 1° C.; however, these waxes do not possess other properties necessary for use with manufactured seeds. For example, manufactured seeds of the kind described herein use a cellulose containing portion, such as a paper straw. The wax composition is applied to the cellulose straw by one of a variety of suitable application processes. Application of the wax composition requires a wax with a viscosity unique to the specific application process utilized. Feasible application processes range from dipping the paper straw within melted wax to spraying the wax thereon. Additionally, manufactured seeds must be able to maintain a degree of rigidity and the wax used with the manufactured seed coat must be able to stay solid (i.e., not melt) under ambient conditions which can be quite hot, especially when the seed is on a slope exposed to direct sunlight. Thus, the wax composition usable with manufactured seeds must possess a melting point and viscosity suitable for use in connection with a manufactured seed coat.
Based on the foregoing, there is a need for an improved seed coat for manufactured seeds having the ability to resist microcracking caused by exposure to cold temperatures. Additionally, there exists a need for a wax formulation for use with manufactured seed coats that has properties consistent with such a use (i.e., an appropriate melting point to resist flow, and an appropriate viscosity for either manual or automated application) and can permit the manufactured seed coat to resist cracking at temperatures below about 1° C.